Four wheel drive vehicles having a transfer case in the driveline for distributing power to the front and rear drive axles are known in the art. In some applications, a bevel gear differential which evenly splits the drive torque between the drive axles is used in the transfer case to drive the front and rear axles at all times, yet allow relative rotation between the axles to accommodate steering geometry. The use of a gear differential in a drive train has one serious drawback. That is, if any wheel of the vehicle is on a low traction surface, the various axle and transfer case differentials allow that wheel to turn freely, and little power or torque is delivered to the remaining wheels.
To minimize wheel slippage, the transfer case differential is usually equipped with a manually operated lock-up mechanism. Such a mechanism is operated in either a locked or unlocked condition. When locked, the mechanism connects the front and rear drive shafts together and positively drives them both. Such a locking mechanism does not allow any differentiation between front and rear drive axle turning speeds. In the event of wheel spin, however, a simple lock up device can cause the previously gripping set of wheels to lose traction.
Several systems have been devicsed to shift a vehicle from two wheel drive to four wheel drive in response to wheel slip conditions. Such systems do not include a center differential or employ the concept of full time four wheel drive. Similarly, there are known systems which shift a four wheel drive vehicle to two wheel drive in response to a steering sensor. Again, such a system does not afford the benefits of full time four wheel drive. U.S. Pat. Nos. 4,484,653; 4,417,641; and 4,558,414 illustrate and describe examples of such systems.
Thus, there remains a need for a transfer case which can deliver full-time four wheel drive but which can respond to changing vehicle conditions. None of the previous attempts offer this unique combination of benefits.